The invention relates to a method for operating the air circulation and conveying stock flow in the housing of a hammer mill which preferably comminutes old material. The invention further relates to a housing of the hammer mill for carrying out the method.
It is generally known that hammer mills can be used for comminuting scrap metal, such as auto bodies, so-called white goods, as well as electronic scrap material and the like.
Hammer mills of this type consist essentially of a housing with a stock inlet, an upper and a lower classifying grate, a rotor rotatably which is supported in the housing and provided with tools, such as hammers, and which is driven with a relatively a high circumferential velocity. The rotor rotating in the housing, however, causes the hammer mill to have attributes of a radial blower that are undesirable in practice. The hammers in this case act as blades which draw in the ambient air through a portion of the grate openings in the lower classifying grate and blow out the air through the grate openings in the upper classifying grate.
This effect causes an overpressure inside the housing, which tries to equalize with the atmospheric pressure. Depending on the composition of a stock to be comminuted, this can generate significant quantities of dust and adversely affect the environment in the vicinity of the hammer mill.
As described in U.S. Pat. No. 4,542,856, it has been observed in hammer mills connected with dust exhaust devices that the blower effect of the hammer rotor has a much greater effect on the size and the air capacity of the dust exhaust device than the actual quantity of dust to be removed.
To optimize the oversized dust exhaust devices, which consume a large amount of energy and are also very noisy and expensive to built, it was suggested in U.S. Pat. No. 4,542,856 A1 to reduce the disadvantageous blower effect of the hammer rotor by subdividing the housing into at least two chambers which are arranged a series in the rotation direction of the rotor. Although this solution appeared to be effective, it was not successful in practical applications, because it was recognized that it is essentially impossible to eliminate the aforedescribed blower effect, leaving little choice but to continue using the expensive, oversized and environmentally harmful dust exhaust devices.
It is therefore an object of the invention to take advantage of the intrinsically disadvantageous blower effect for operating the hammer mill, to design the housng accordance the complexity of the required dust exhaust devices.
According to the method of the invention, the object is solved in that the air flow and the stock flow, after having passed the upper classifying grate, is diverted by a deflector hood and a subsequent duct downwardly in an essentially vertical direction and offset as well as separated from the housing wall to the region of the lower classifying grate. The existing underpressure causes the air fraction to be again drawn into the comminution chamber after passing the lower classifying grate, thereby establishing a continuous recirculating air flow with an approximately constant air volume.
If a dust exhaust device is connected, the air capacity of the dust exhaust device can be reduced by the fraction of the air that is already circulating.
Advantageously, the connection for the dust exhaust air can be moved to the end of the channel and to the region of the lower classifying grate, respectively, to improve the sorting effect for removing the comminuted light stock, for example upholstery materials (light shredder fraction), from the comminuted heavy stock, for example comminuted metal pieces, such as old car bodies, that are conveyed from the hammer mill through the stock inlet.
The sorting effect can be improved by providing the device, which is located in the region of the lower classifying grate and intended for further processing, sorting and/or conveying of the comminuted old material, with a stage/cascade, into which an additional blower having a lower power blows in air. In this way, the lighter components, for example the so-called light shredder fraction, can be separated from the metal fraction more precisely before being conveyed to the dust exhaust duct.
An additional effect can be achieved by adding during the comminution process finely atomized or nebulized water to the circulating air flow and/or into the housing, whereby the water molecules and/or the large surface area of the water particles extract a significant amount of heat from the supplied old material, for example the car bodies, during the comminution process. This also reduces the danger of an explosion and fire as well as the creation of burnt hydrocarbons or so-called xe2x80x9cblue smokexe2x80x9d, which is frequently generated by residual fuel or drawing compounds that remain in the old material.
Nebulized water is preferably injected into the housing above the rotor through tubes. The quantity of nebulized water per unit time can be controllably supplied as a function of the quantity of material in the comminution chamber by measuring the electric current drawn by the drive motor that drives the rotor or by measuring the temperature difference between the internally circulating air and the ambient air.
The housing of the hammer mill is connected with a dust exhaust device. The dust exhaust air volume is reduced by a fraction of the air volume produced by the blower effect of the rotor, and is added in a region of a lower classifying grate and removed together with portions of a comminuted fraction to the dust exhaust device through a dust exhaust duct which is separate from the duct. A portion of the admitted dust exhaust air together with fine particles of the scrap material is removed to the dust exhaust device in the region of the stock inlet. The stock flow that is combined in the region of the lower classifying grate is supplied with additional air through a blower via a cascade , whereby light fraction components of the comminuted scrap material are conveyed through the dust exhaust duct to the dust exhaust device. During the comminution process finely atomized or nebulized water is supplied to the circulating air flow and/or the housing or the comminution chamber, whereby heat is removed from the supplied scrap material by the water molecules during the comminution and/or the danger risk of an explosion or fire as well as the formation of burnt hydrocarbons is at least reduced. The finely atomized or nebulized water with the dimension quantity vs. unit time is added and regulated as a function of the quantity of material in the comminution chamber by measuring the electric current drawn by the drive motor that drives the rotor or by measuring the temperature difference between the internally circulating air and the surrounding air.
An upper classifying grate is used, wherein a plane of the grate is inclined relative to the rear wall of the housing, and wherein the stock flow is guided between grate bars that are oriented perpendicular to the inclined grate plane. An upper classifying grate is used, with a plane of the grate being oriented perpendicular relative to the rear wall of the housing or horizontally, and wherein the stock flow is guided between grate bars that are inclined with respect to the horizontal. A pivotable upper classifying grate may be used. Also, a controller for conveying the stock flows is used to which the circulating air or dust exhaust air is applied, depending on the composition of the old material, the composition and type of the comminuted material, as well as the quantity, power and/or pressure of the circulating air or dust exhaust air to be produced.
The housing of a hammer mill for carrying out the method, includes a rotor which is rotatably supported in a housing and provided with tools, a stock inlet, a stock outlet provided in the upper housing with an upper classifying grate and a stock outlet provided in the lower housing with a lower classifying grate. The housing has a deflection hood following the upper classifying grate, followed by a duct leading to the region of the lower classifying grate. A dust exhaust duct is provided which extends parallel to the duct to a dust exhaust device. The dust exhaust duct is located after the duct in the region of the lower classifying grate and deflects a portion of the air flow and conveys portions of a comminuted fraction with the dust exhaust air. The region performing the deflection from the duct to the dust exhaust duct as well as at least a portion of the region of the lower classifying grate are implemented as a device for further sorting of the comminuted old material. A blower is associated with the device, the blower in conjunction with a stage/cascade disposed in the flow of the device representing a sorter for effectively separating the light fraction components of the comminuted scrap material and conveying the light fraction components via the dust exhaust duct to the dust exhaust device. A swing-out door is provided. The upper classifying grate is inclined as it covers the upper stock outlet, wherein the grate rods, as viewed in cross-section, are oriented perpendicular to the inclination plane. The upper classifying grate is horizontal as it covers the upper stock outlet, wherein the grate rods, as viewed in cross-section, are inclined relative to the horizontal. A pivotable upper classifying grate which at the same time represents a counter-tool for the tools of the rotor as well as a swing-out door.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are intended solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims.